CD8+ T cells (TCD8+ ) play an important role in controlling virus infections. TCD8+ recognize peptides of 8 to 10 residues derived from viral proteins located in the cytosol of virus infected cells. These peptides are recognized in a complex with class I molecules encoded by the major histocompatibility complex (MHC). In the past 3 years it was discovered that the MHC also encodes two molecules, termed TAP1 and TAP2, that seem to specifically transport peptides from the cytosol into the intracellular compartment that contains class I molecules. TAP genes have been found to be polymorphic in humans, although to a far lesser extent than the class I molecules. The existence of these putative peptide pumps and their polymorphism raises a number of important questions: Where in the cells are the pumps located? Do the pumps influence the types of peptides presented by class I molecules? Are the pumps tethered to the proteases that produce antigenic peptides in the cytosol? How do the pumps work? Are there individuals with immune deficiencies based on mutations in the TAP genes? Can cells transport peptides via other mechanisms? To help characterize the structure and function of the TAP genes we have inserted them into vaccinia virus. We have demonstrated that a Vac recombinant encoding TAP1 produces a fully functional gene, and have found that TAP1 produced by this recombinant localizes to the endoplasmic reticulum where it is glycosylated. We produced a recombinant that produces a functionally active TAP2. We have also examined the specificity of the antigen transporters. We found that the transporters can transport the exact peptides presented by class I molecules. Also we examined whether expression of the human TAP2 gene in mouse cells adversely affects the ability of the cells to present viral proteins to TCD8+. We find that the human TAP2 gene functions in a indistinguishable manner from its mouse counterpart. Finally, we have explored the presentation of peptides in cells that fail to express one or both TAP genes.